This invention relates to a variable focal length lens of the type called a zoom lens which is used for photographic and TV cameras, and particularly to a variable focal distance lens equipped with a movable focusing lens group suitable for a lens system having a wide angle of field.
More specifically stated, the invention relates to a variable focal distance lens system comprising three parts including a zooming part consisting of a plurality of movable lenses which are provided for continuously varying the focal distance of the whole lens system within a preset range while the location of an image surface for any desired object is kept unvaried; a focusing part which is disposed ahead of the zooming part (on the object side) and serves to provide an object point (an image point of a photographing object) unvarying relative to the zooming part irrespective of variation in the distance to the object; and an image forming lens part (relay lens) which is disposed behind the zooming part (on the image side), the lens system being arranged to satisfactorily correct aberration at every distance to objects and at every zoom position.
It is a recent requirement to make shorter the short focal distance side in the variable focal distance range of a zoom lens for a wider angle of field. To meet such requirement, the lens aperture of the focusing part must be increased. Besides, the construction of the lens becomes complex. In other words, in order to secure an effective diameter required, the outer diameter of the lens must be increased; furthermore, aberration correction is required not only over the magnification changing range but also covering the whole range of focusing from an infinite distance to a short distance; and this results in a complex lens construction. Thus, the ratio of the lens weight of the focusing part to the whole lens system greatly increases. In addition to such, compared with the conventional arrangement, the weight of the mechanism to be used for moving the lens group for focusing also becomes very heavy. Such problems present difficulty in making the lens system smaller and lighter. In view of such difficulty, there have been known lens systems of the type which, instead of drawing out a whole front lens part, is composed of a movable part for focusing and a fixed part for aberration correction. Such known systems include a system disclosed in British Pat. No. 975160 wherein the front lens part is composed of a negative lens group and a positive lens group and only the negative lens group is moved on the optical axis; and a system disclosed in U.S. Pat. No. 3598476 wherein the front lens part is composed of a negative lens group, a positive lens group and another negative lens group and only the positive lens group which is disposed in the middle is moved toward an object as the distance to the object becomes shorter.